DE 44 40 613 C1 teaches the detection and demodulation of intensity modulated wavefields with sensing elements that consist of three parts: one photosensitive part, in which incident photons are converted into a proportional number of electronic charge pairs, one or more storage elements, into which the photogenerated charges are stored and accumulated and an equal number of switches between the photosensitive part and each storage element. The switches are operated synchronously with the modulation frequency. A preferred embodiment relies on charge coupled device (CCD) techniques, as described by A. J. P. Theuwissen in “Solid-state imaging with charge-coupled devices”, Kluwer, Dord-recht, 1995. There the photosensitive site and the switches are realized and operated as CCD gates that transport the photogenerated charge laterally. Disadvantages of this approach include the limited demodulation speed that is obtained with CCDs, especially if large photosensitive sites and CCD gates are employed, the necessity for special semiconducting processes for the fabrication of the CCD structures, and the demands on clocking waveforms with specially shaped rising or falling edges in order to obtain a high charge transfer efficiency under the CCD gates.
An alternative embodiment of the switches employs field effect transistors. (FETs), as available in industry standard Complementary Metal Oxide Semiconductor (CMOS) processes. This type of switch is simpler to operate, and it is readily fabricated. The disadvantage of the FET switch is increased charge and voltage noise behaviour due to incomplete charge transfer, charge injection effects and channel current noise caused by gate voltage fluctuations.
DE 198 21 974 A1 claims to overcome the speed limitations of large photosensitive elements by replacing the single large photogate with a comb-like structure of interdigitated finger electrode photogates. The photogenerated charge carriers are therefore more rapidly collected, and they can also be transferred more quickly on two or more storage elements. This invention relies also on switching elements for transferring photocharge onto suitable storage elements. The disadvantages of these switching elements, realized as CCD gates of FETs, are the same as described for DE 44 40 613 C1.
EP 00109721 describes an alternative sensing element for the detection and demodulation of intensity modulated wavefields. It employs two photosensing parts per sensing element, each with two storage sites and associated switching element. When used in conjunction with a diffusing optical component on top of the sensing element for the equal distribution of the incoming wavefield intensity on the two photosites, this device allows prolonged integration times and relieves the timing restrictions on the clock waveform. The number of storage sites is limited to four, rendering this device ineffective if more than four samples per period of the modulated waveform should be taken. Since this invention also relies on switches for the transfer of photocharges from the photosites to the storage elements the same disadvantages are encountered as described for the above two inventions.
CH 3176/96 teaches the use of a resistive, elongated electrode with a static voltage difference at the two ends, as a means for photogenerating and transporting charge carriers with improved speed along one lateral direction. This is achieved with the static lateral electric field that is created parallel to the surface at the semiconductor-insulator interface. This lateral field moves photocharges significantly faster compared with a conventional CCD structure that has an electrode of the same size but not resorting to the lateral electrical field disclosed in this invention. Since photocharge can only be moved in one fixed direction, no demodulation action for an incident modulated wavefield can be obtained with such a device.
U.S. Pat. No. 5,528,643 describes even faster lateral transport of photogenerated charge carriers, by employing a series of CCD gates, each of which has contacts at both ends at which voltage differences can be applied. In this way, each CCD electrode exhibits a lateral drift field at the semiconductor-insulator interface. The object of the invention disclosed in U.S. Pat. No. 5,528,643 is the architecture of a two-dimensional CCD image sensor with improved photocharge transport speed in the column and read-out line directions. Since photocharge can only be moved in one fixed direction, no demodulation action for an incident modulated wavefield can be obtained with such a device.